The present invention relates to a filming composition for a cathode ray tube and a method for manufacturing a screen using the same, and more particularly, to a composition for forming a filming layer as an interlayer between a phosphor layer and a metal deposited layer, and a method for manufacturing a screen having enhanced luminance using the same.
In manufacturing the screen of a general cathode ray tube, a black matrix layer made of graphite is formed on the inner face of a panel and on the portion where pixel will not be formed. A phosphor layer for forming a pixel is formed on the black matrix layer using a photoresist formed by photolithography and a filming layer is formed as an interlayer on the phosphor layer by coating or spraying a filming composition. A metal layer is formed on the filming Layer by depositing metal.
Thereafter, the interlayer, i.e., the filming layer formed between the phosphor layer and the metal layer, is removed by heating to a high temperature. As the result, the metal layer is provided on the phosphor layer with a predetermined distance therebetween. The reason for forming the metal layer at a distance from the phosphor layer will be explained in detail, referring to the attached drawings.
When electrons emitted from an electron gun collide with the phosphor particles in the phosphor layer 3, the phosphor particles emit light in all directions, including both the front side and back side of the panel 1. At this time, the metal deposited layer S such as aluminum-deposited layer reflects the light emitted to the back side of the panel 1 to the front side to enhance the luminance at the front side of the screen. However, if the metal deposited layer 5 is formed directly on the phosphor layer 3, the metal is injected between the phosphor particles and a uniform and continuous layer cannot be obtained (FIG. 1). Therefore, the light radiates through the non-continuous point of the layer to reduce the reflection amount, and the luminance at the front side of the screen cannot be sufficiently enhanced.
To solve this problem, an organic layer (referred to as a filming layer 4) which can be decomposed by heating is formed on the phosphor layer 3 and a metal deposited layer 5 is formed on the filming layer (FIGS. 2 & 3). Subsequently, through removing the organic material by heating to a temperature at which the organic layer is decomposed, a flat and uniform metal layer spaced apart from the phosphor layer by a predetermined distance is manufactured.
Two methods, an aqueous system and an oily system, are used for manufacturing the filming layer between the phosphor layer and the metal layer. In the aqueous system, a filming composition comprising acryl emulsion as a main component is used. On the other hand, lacquer is used in the oily system (thus called a lacquer method).
In the aqueous system, the main components of the filming composition are polyvinyl alcohol and acryl emulsion. To these components can be added glycerine as a wetting agent and ammonium hydroxide and hydrogen peroxide to prevent swelling of the metal layer during baking. In the aqueous system, separate baking at a temperature of 400.degree..about.450.degree. C. is needed after forming the metal layer in order to decompose and remove the organic materials in the phosphor layer and the filming layer. However, frit sealing at 400.degree..about.450.degree. C. for sealing a panel and a funnel should be carried out afterward. As a result, the metal layer is baked twice.
Of course, the organic material may be decomposed and removed during the frit sealing process. However, in this case the following problems occur. The acryl emulsion, B74 (by Rohm & Haas), used as a main component in the conventional filming composition has a molecular weight of about 4 million and a decomposition starting temperature of 300.degree. C. or higher. If the separate baking for decomposing the organic material is not carried out, since the decomposition of the organic material in the filming layer is delayed and organic gas remains even at temperatures of 400.degree..about.450.degree. C., which is the softening and crystallizing point of the frit. The residual gas inhibits the frit sealing so that complete sealing is difficult. Even when sealing is done, since organic material remains due to the incomplete combustion or the residual gas remains in the cathode ray tube, the characteristics of the cathode ray tube such as lifetime and luminance are deteriorated. Accordingly, separate baking to decompose and remove the organic material cannot be omitted.
In the oily system (the lacquer method), a lacquer prepared by dissolving acryl resin in a solvent such as toluene, ethyl acetate, or methyl ethyl ketone is sprayed on the phosphor layer to obtain a filming layer. In this method, the surface tension of the organic solvent with water is utilized. That is, the organic material is dispersed to form a thin organic layer on the water layer. Accordingly, the filming layer is very flat while having a very small amount of organic material. Thus, a separate baking to remove the organic material can be advantageosly omitted. After forming the filming layer and the metal layer by depositing metal such as aluminum, the remaining small amount of organic material can be almost completely removed through the frit sealing.
However, in this method, since the organic solvent is volatile and explosive, separate equipment, such as a ventilation system and a closed room, is needed and the process is overly complicated, so the maintenance and management of the process is very difficult.
The aqueous system and the oily system, each having some advantages and disadvantages, both contain a common problem. That is, the metal layer, especially the aluminum layer formed on the filming layer, is oxidized by heat treatment which is performed at least once.
If the metal layer is oxidized, the light reflection ratio is abruptly reduced, and as the result, the intensity of the light toward the phosphor layer is reduced to deteriorate the luminance of the screen. Luminance is an important factor for determining the image quality of a cathode ray tube, and affects the design of the shadow mask and electron gun. Accordingly, it is considered essential to avoid such luminance deterioration.